What is a Swimming Pool?
In times past people used to swim and bathe in natural waters. Roman emperors and kings of other countries had private pools in which they kept fish. The first indoor swimming pools in England were constructed early in the 19th century. Largely untreated, early pools got dirtier as the days went by and had to be emptied and refilled as required.
When charging was brought in, the price of a dip might fall as the water got dirtier. A way around the problem of pollution was to build pools that water flowed through on a regular and continuous basis, usually from a nearby river or stream.
These days all but the humblest pool is quite a bit more complex than that.
How Swimming Pools Work
Swimming pools come in all shapes and sizes, but nearly all of them, from the domestic to the Olympic, from the rectangular to the variously shaped leisure, work in the same fundamental way. Anything with more than about 5 cubic metes of water normally uses a combination of filtration and chemical treatment to clean continually by re-circulating a large volume of water.
A typical swimming pool needs four major components:
A tank (basin, shell)
A circulation system – pumps, inlets and outlets, pipe work
A dosing system for treatment chemicals
The basic idea is to pump water in a continual cycle, from the pool via filtration and chemical treatment and back to the pool again. By this circulation, the water in the pool is kept relatively free of dirt, debris and micro-organisms (bacteria and virus). Other processes are included – heaters, for example. It must be possible to make up water lost by evaporation, back-washing the filters and bathers carrying it out of the pool on costumes etc.
Types of Swimming Pool Construction
Concrete Swimming Pools
Concrete pools are usually finished in mosaic tiles. As these are not flexible, the pool shell must be reinforced concrete, designed as a monolithic structure by a structural engineer and fit to be built in the geological conditions of the site chosen.
The standards for reinforced concrete structures is, British Standard 8110-1:1997 Structural use of concrete. British Standard 8007:1987 Design of concrete structures for retaining aqueous liquids is an adjunct to BS8110.
Generally speaking the vast majority of commercially built swimming pools and a small percentage of domestic pool tanks are built to BS 8007. This requires that the pool tank is inherently watertight [strictly, up to 10mm loss per week]. It does not rely on waterproof renders or applied finishes to stop leaking. It must be water tested after construction and before the internal finishes are applied to confirm this.
Pool tanks built to BS8110 are steel reinforced structures but are not themselves watertight; this is achieved by the application of a waterproof render and screed.
The method of construction can vary depending on the structural engineer or pool designer, the capabilities of the pool builder and cost considerations.
Generally speaking pools built to BS8007 are more expensive due to the higher degree of civil engineering. The general methods of construction are permanent shuttering, temporary shuttering, hollow concrete blocks and sprayed concrete.
There are a whole range of surface finishes which can be applied either direct onto the concrete, or onto renders applied to the concrete when a smooth true surface or fine dimensional tolerance is important. The most common finish used is a ceramic tile or mosaic which will give a long service life and should be designed and applied in accordance with British Standard 5385 wall and floor tiling, with particular reference to part 4 which covers the application of ceramics in wet or submerged conditions.
PRE-FORMED SECTIONAL [PFS] POOLS
Over the last 25 years or so, the design and installation of PFS pools have developed substantially. They can be made from a range of materials including welded stainless steel plate, stainless steel, galvanised panels and glass reinforced plastic [GRP] panels that are bolted together. Water retention is achieved with a heavy- duty uPVC membrane.
These quick fix pools are considered to be a cheap option, but now are nearly as much as a concrete pool, why would you have one?
VINYL-LINED IN-GROUND POOLS
The vinyl-lined pool has no structural strength to speak of, as it is watertight only because of its uPVC flexible liner. The structure’s wall assembly can be block work, preformed plastic, composite or galvanised tin panels or even timber. The floor is porous to allow the ground water pressure to move the liner and push into the pool, thus preventing the pool assembly being distorted. Again UK swimming pools view these pools as cheap and nasty options, although the prices now are almost the same as a bespoke concrete pool, why would you have one?
ABOVE GROUND POOLS
These can be the cheapest construction option, as well as the easiest to build. Most above ground pools are made from prefabricated kits, designed for an amateur to put together (though some people use professional installers). The main disadvantage of this sort of pool is that it’s less durable than other designs, and generally less attractive. Their impermanence can be a good thing: it’s relatively easy to disassemble the pool and move it to a new location. Large ones have been used in public swimming competitions.
FULL AND EMPTY
A concrete in-ground swimming pool may seem like a solid, immovable structure, but it is actually rather like a boat: it can float where the surrounding ground water table is high. Unless the tank has been designed to withstand uplift when the pool is empty the ground water pressure can actually push the pool up out of the ground.
Partly in order to resist this pressure, in-ground pools are generally kept full all year round. Many in-ground pools have special hydrostatic relief valve near the main drain to guard against ground water pressure. Basically if the ground water pressure is great enough, it will push up on a small float, which opens the valve. When the valve opens, ground water flows into the main drain and equalises the pressure. The seal on this type of valve needs regular careful maintenance to keep it free of grit and silt and stop pool water leaking into the surrounding ground.
This approach to equalising the pressures will restrict the opportunity for maintenance to times when the water table is at its lowest, which unfortunately is likely to be mid summer, when the pool is most in demand.
Care must be taken emptying or filling pools. A tiled pool must be filled or emptied at a constant rate no greater than 750mm in 24 hours and the water heated at a constant rate no greater than 0.25 degrees C per hour to avoid thermal shock.
The most common leakage problem in small pools is when pipework and fittings [skimmers, underwater lights, main drain etc] run through the walls and the floor of the pool tank. So great care needs to be taken to ensure a good watertight joint at these interfaces.
OUTLETS AND INLETS:
During normal operation, pool water is removed at the bottom of the pool through two or more main drain outlets and from the surface perimeter of the pool into deck-level transfer channels, skimmers or overflow channels. The bottom outlets are usually on the lowest point in the pool, so that the entire pool floor surface slopes toward them. Most of the dirt and debris, heavier than water, will sink and leave the pool through these outlets. Some debris will usually have to be vacuumed up. The surface water draw off system works the same way as the main outlets, but they should only take water from the very top of the pool. As a result, any pollution that more or less floats-skin cells, hair etc- leaves the pool from the surface. Today most public swimming pools/spas work on the deck-level principle: up to 80% of water removal is actively drawn from the surface of the pool, overflowing via transfer channels round the pool. Inlets and outlets also need to be safely designed, with a safe water flow rate. The designer specifies the number and arrangement needed to achieve the recommended flow rates. To keep people from getting their hair and limbs caught in the pipework, the outlets are covered with grilles or special covers. Water is draw out of the pool by circulation pumps through the pool outlets, and is returned to the pool via the filtration and treatment systems, through the pool inlets.
The heart of the swimming pool system is the water circulation pump. In a typical swimming pool one or more electric pumps will draw water from the pool, pull it through a strainer and push it through the filtration system and back into the pool. At appropriate points chemicals are dosed to treat the pool water.
Sand filters are the most common type of filter used today for swimming pools, spas and sometimes ponds The filter system consist of a ‘keg’ or tank, and a multidirectional valve , which changes the way the water passes through the sand filled tank/keg. During filtration, dirty water from the pool is pumped down through the filter bed and the tiny sand particles capture the many insoluble pollutants that can be filtered. At the bottom of the tank, the filtered water flows through the collection system and out the outlet pipe. Over time, the debris collected in the sand slows down the water flow. If gauges show an increase in pressure on the inlet pipe compared to the outlet pipe , the operator knows there’s a lot of collected debris in the sand. This means its time to backwash [cleanse] the filter. To backwash, the operator [after switching off the pump!] turns the multidirectional valve to ‘backwash’ then turns the pump back on , and water flows the opposite way through the sand taking all the debris to the waste pipe/hose. After a while, the pump is switched off again and the valve replaced to the filter setting. The pump is switched on again and normal filtering continues.
The swimming pools filter does the hard work in keeping the water clean, but it takes chemistry to make sure the pool water is safe and suitable to swimming. Failure to maintain appropriate chemical concentrations in the pool water can affect users health and damage parts of the pool and its associated equipment.